The collapse of large parts of the ice sheet in West Antarctica appears to have begun and is almost certainly unstoppable, with global warming accelerating the pace of the melting, two groups of scientists reported Monday.

The finding, which had been feared by some scientists for decades, means that a rise in global sea level of at least 3 metres (10 feet) may now be inevitable. The rise may continue to be relatively slow for at least the next century or so, the scientists said, but sometime after that it will probably speed up so sharply as to become a crisis.

"This is really happening," said Thomas P. Wagner, who runs NASA's programs on polar ice and helped oversee some of the research. "There's nothing to stop it now. But you are still limited by the physics of how fast the ice can flow."

Two papers scheduled for publication this week, in the journals Science and Geophysical Research Letters, attempt to make sense of an accelerated flow of glaciers seen in parts of West Antarctica in recent decades.

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Both papers conclude that warm water upwelling from the ocean depths has most likely triggered an inherent instability that makes the West Antarctic ice sheet vulnerable to a slow-motion collapse. And one paper concludes that factors some scientists had hoped might counteract such a collapse will not do so.

The new finding appears to be the fulfillment of a prediction made in 1978 by an eminent glaciologist, John H. Mercer of the Ohio State University. He outlined the uniquely vulnerable nature of the West Antarctic ice sheet and warned that the rapid human release of greenhouse gases posed "a threat of disaster." He was assailed at the time, but in recent years scientists have been watching with growing concern as events have unfolded in much the way Mercer predicted. (He died in 1987.)

Scientists said the ice sheet was not melting because of warmer air temperatures but rather because of the relatively warm water, which is naturally occurring, from the ocean depths. That water is being pulled upward and toward the ice sheet by intensification of the winds around Antarctica.

Most scientists in the field see a connection between the stronger winds and human-caused global warming, but they say other factors are likely at work, too. Natural variability of climate may be one of them. Another may be the ozone hole over Antarctica, caused by an entirely different environmental problem: the human release of ozone-destroying gases.

Antarctic ice sheets are melting as warmer waters erode them from below.

Whatever the mix of causes, they appear to have triggered a retreat of the ice sheet that can no longer be stopped, even if the factors drawing in the warmer water were to reverse suddenly, the scientists said. At this point, a decrease in the melt rate back to earlier levels would be "too little, too late to stabilise the ice sheet," said Ian Joughin, a glaciologist at the University of Washington and lead author of the new paper in Science. "There's no stabilisation mechanism."

The basic problem is that much of the West Antarctic ice sheet sits below sea level in a kind of bowl-shaped depression. As Mercer outlined in 1978, once the part of the ice sheet sitting on the rim of the bowl melts and the ice retreats into deeper water, it becomes unstable and highly vulnerable to further melting.

Richard B. Alley, a climate scientist at Pennsylvania State University who was not involved in the new research but has studied the polar ice sheets for decades, said he found the new papers compelling. Although he has long feared the possibility of ice-sheet collapse, when he learned of the new findings, "it shook me a little bit," Alley said.

He added that while a large rise of the sea may now be inevitable from West Antarctica, continued release of greenhouse gases will almost certainly make the situation worse. The heat-trapping gases could destabilise other parts of Antarctica as well as the Greenland ice sheet, causing enough sea-level rise that many of the world's coastal cities would eventually have to be abandoned.